In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
During machining of steels, stainless steels and cast irons with coated cemented carbide tools, the cutting edges are worn according to different wear mechanisms, such as chemical wear, abrasive wear and adhesive wear. At high cutting speeds, the amount of heat generated in the cutting zone is considerable and a plastic deformation of the cutting edge may occur, which in turn yields an enhanced wear by other mechanisms. During milling operations in alloyed steels, adhesive wear is often pronounced and edge chipping occurs frequently as a consequence of delamination or cracking of the protective coating.
The cutting performance with respect to specific wear types can be improved by single actions, however, very often this will have a negative effect on other wear properties. Consequently, successful tool composite materials must be designed as careful optimizations of numerous properties. In the case of milling of alloyed steels and tool steels, which are often in a hardened state, one important balance is between plastic deformation resistance of the cutting edge and edge chipping resistance. A simple measure to increase the resistance to plastic deformation and also the abrasive wear resistance is to lower the binder phase content. However, this will also diminish the toughness of the cutting insert, which can substantially lower the tool life in applications where vibrations or the presence of casting or forging skin put demands on such properties. An alternative way to increase the deformation resistance is to add cubic carbides like TiC, TaC and/or NbC. However, this addition has a negative influence on edge chipping tendencies and so called comb crack formation. The constitution of the applied wear resistant surface coating is a key factor in the properties of the tool. Thicker and more wear resistant coatings are often applied by the chemical vapor deposition (CVD) method. These coatings often also improves the plastic deformation resistance but can to larger extent impair edge toughness. Coatings produced by physical vapor deposition (PVD), which are often thinner, do not provide as good protection against heat and plastic deformation but give very good edge integrity and consequently shows good protection against edge chipping.
To improve all tool properties simultaneously is very difficult and numerous properties of both the protective coating and the cemented carbide substrate and the combination thereof have to be considered. Consequently, commercial coated cemented carbide grades have usually been optimized with respect to one or a few wear types. This also means that they have been optimized for only specific applications.
U.S. Pat. No. 6,062,776 discloses a coated cutting insert particularly useful for milling of low and medium alloyed steels and stainless steels with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces under unstable conditions. The insert is characterized by a WC—Co cemented carbide with a low content of cubic carbides and a rather low W-alloyed binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains, a top layer of TiN and an inner layer of κ-Al2O3.
U.S. Pat. No. 6,177,178 describes a coated milling insert particularly useful for milling in low and medium alloyed steels with or without raw surface zones during wet or dry conditions. The insert is characterized by a WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiCxNyOz with columnar grains, an inner layer of κ-Al2O3 and, preferably, a top layer of TiN.
U.S. Pat. No. 6,250,855 provides a coated cemented carbide cutting tool for wet and dry milling of stainless steels at high cutting speeds. The tool has a cemented carbide body comprising a substrate based on WC—Co without any additions of cubic carbides. The coating includes a very thin layer of TiN, a second layer of (Ti,Al)N with a periodic variation of the Ti/Al ratio and an outermost layer of TiN.
WO 01/16389 discloses a coated milling insert particularly useful for milling in low and medium alloyed steels with or without abrasive surface zones during dry or wet conditions at high cutting speed, and milling hardened steels at high cutting speed. The insert is characterized by WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an innermost layer of TiCxNyOz with columnar grains and a top layer of TiN and an inner layer of κ-Al2O3.
EP 1103635 provides a cutting tool insert particularly useful for wet and dry milling of low and medium alloyed steels and stainless steels as well as for turning of stainless steels. The cutting tool is comprised of a cobalt cemented carbide substrate with a multi-layer refractory coating thereon. The substrate has a cobalt content of 9.0-10.9 wt % and contains 1.0-2.0 wt % TaC/NbC. The coating consists of an MTCVD TiCxNyOz layer and a multi-layer coating being composed of κ-Al2O3 and TiCxNyOz layers.